opencv: VideoCapture can't open camera by index

I faced this issue and got it resolved in my case: [ WARN:0] global /tmp/pip-req-build-ms668fyv/opencv/modules/videoio/src/cap_v4l.cpp (893) open VIDEOIO(V4L2:/dev/video0): can't open camera by index I was trying to live stream webcam video on local host server.

Here is my error code:

`

from flask import Flask, render_template, Response import cv2

app = Flask(name) cap = cv2.VideoCapture(0) # wrong position, error def igen_frames():

while(True):
	ret, frame = cap.read()
	gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
	cv2.imshow('frame',gray)
	if cv2.waitKey(1) & 0xFF == ord('q'):
	    break
cap.release()
cv2.destroyAllWindows() 

Here is my resolved code:

` from flask import Flask, render_template, Response import cv2 app = Flask(name) def igen_frames(): cap = cv2.VideoCapture(0) #resolved, correct position

while(True):
	ret, frame = cap.read()
	gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
	cv2.imshow('frame',gray)
	if cv2.waitKey(1) & 0xFF == ord('q'):
	    break
cap.release()
cv2.destroyAllWindows() `

As you can see, it was resolved when cap = cv2.VideoCapture(0) was inside the function. So make sure that you place "cap = cv2.VideoCapture(0) " in a correct position if you are reading the frames inside a function.

Try to specify videoio backend:

-VideoCapture capture(0);
+VideoCapture capture(CAP_V4L2);

or disable GStreamer backend in runtime or during the build.

[ WARN:0@0.332] global /io/opencv/modules/videoio/src/cap_v4l.cpp (902) open VIDEOIO(V4L2:/dev/video0): can’t open camera by index. This is the error i have when running python demo.py on WSL ubuntu , python 3.6, opencv 4.6

Below is the code.

‘’‘MIT License Copyright © 2020 Prokofiev Kirill, Intel Corporation Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.’‘’

import argparse import inspect import os.path as osp import sys

import cv2 as cv import glog as log import numpy as np

current_dir = osp.dirname(osp.abspath(inspect.getfile(inspect.currentframe()))) parent_dir = osp.dirname(current_dir) sys.path.insert(0, parent_dir) import utils from demo_tools import TorchCNN, VectorCNN, FaceDetector

def pred_spoof(frame, detections, spoof_model): “”“Get prediction for all detected faces on the frame”“” faces = [] for rect, _ in detections: left, top, right, bottom = rect # cut face according coordinates of detections faces.append(frame[top:bottom, left:right]) if faces: output = spoof_model.forward(faces) output = list(map(lambda x: x.reshape(-1), output)) return output return None, None

def draw_detections(frame, detections, confidence, thresh): “”“Draws detections and labels”“” for i, rect in enumerate(detections): left, top, right, bottom = rect[0] if confidence[i][1] > thresh: label = f’spoof: {round(confidence[i][1]*100, 3)}%’ cv.rectangle(frame, (left, top), (right, bottom), (0, 0, 255), thickness=2) else: label = f’real: {round(confidence[i][0]*100, 3)}%’ cv.rectangle(frame, (left, top), (right, bottom), (0, 255, 0), thickness=2) label_size, base_line = cv.getTextSize(label, cv.FONT_HERSHEY_SIMPLEX, 1, 1) top = max(top, label_size[1]) cv.rectangle(frame, (left, top - label_size[1]), (left + label_size[0], top + base_line), (255, 255, 255), cv.FILLED) cv.putText(frame, label, (left, top), cv.FONT_HERSHEY_SIMPLEX, 1, (0, 0, 0))

return frame

def run(params, capture, face_det, spoof_model, write_video=False): “”“Starts the anti spoofing demo”“” fourcc = cv.VideoWriter_fourcc(*‘MP4V’) resolution = (1280,720) fps = 24 writer_video = cv.VideoWriter(‘output_video_demo.mp4’, fourcc, fps, resolution) win_name = ‘Antispoofing Recognition’ while cv.waitKey(1) != 27: has_frame, frame = capture.read() if not has_frame: return detections = face_det.get_detections(frame) confidence = pred_spoof(frame, detections, spoof_model) frame = draw_detections(frame, detections, confidence, params.spoof_thresh) cv.imshow(win_name, frame) if write_video: writer_video.write(cv.resize(frame, resolution)) capture.release() writer_video.release() cv.destroyAllWindows()

def main(): “”“Prepares data for the antispoofing recognition demo”“”

parser = argparse.ArgumentParser(description='antispoofing recognition live demo script')
parser.add_argument('--video', type=str, default=None, help='Input video')
parser.add_argument('--cam_id', type=int, default=-1, help='Input cam')
parser.add_argument('--config', type=str, default=None, required=False,
                    help='Configuration file')
parser.add_argument('--fd_model', type=str, required=True)
parser.add_argument('--fd_thresh', type=float, default=0.6, help='Threshold for FD')
parser.add_argument('--spoof_thresh', type=float, default=0.4,
                    help='Threshold for predicting spoof/real. The lower the more model oriented on spoofs')
parser.add_argument('--spf_model', type=str, default=None,
                    help='path to .pth checkpoint of model or .xml IR OpenVINO model', required=True)
parser.add_argument('--device', type=str, default='CPU')
parser.add_argument('--GPU', type=int, default=0, help='specify which GPU to use')
parser.add_argument('-l', '--cpu_extension',
                    help='MKLDNN (CPU)-targeted custom layers.Absolute path to a shared library with the kernels '
                         'impl.', type=str, default=None)
parser.add_argument('--write_video', type=bool, default=False,
                    help='if you set this arg to True, the video of the demo will be recoreded')
args = parser.parse_args()
device = args.device + f':{args.GPU}' if args.device == 'cuda' else 'cpu'
write_video = args.write_video

if args.cam_id >= 0:
    log.info('Reading from cam {}'.format(args.cam_id))
    #cap = cv.VideoCapture(CAP_V4L2)
    cap = cv.VideoCapture(args.cam_id)
    cap.set(cv.CAP_PROP_FRAME_WIDTH, 1280)
    cap.set(cv.CAP_PROP_FRAME_HEIGHT, 720)
    cap.set(cv.CAP_PROP_FOURCC, cv.VideoWriter_fourcc(*'MJPG'))
else:
    assert args.video
    log.info('Reading from {}'.format(args.video))
    cap = cv.VideoCapture(args.video)
    cap.set(cv.CAP_PROP_FOURCC, cv.VideoWriter_fourcc(*'MJPG'))
assert cap.isOpened()
face_detector = FaceDetector(args.fd_model, args.fd_thresh, args.device, args.cpu_extension)
if args.spf_model.endswith('pth.tar'):
    if not args.config:
        raise ValueError('You should pass config file to work with a Pytorch model')
    config = utils.read_py_config(args.config)
    spoof_model = utils.build_model(config, args, strict=True, mode='eval')
    spoof_model = TorchCNN(spoof_model, args.spf_model, config, device=device)
else:
    assert args.spf_model.endswith('.xml')
    spoof_model = VectorCNN(args.spf_model)
# running demo
run(args, cap, face_detector, spoof_model, write_video)

if name == ‘main’: main()

when i run the command based on the argparser argument python3 demo.py --fd_model /home/adesoji/light-weight-face-anti-spoofing-master/light-weight-face-anti-spoofing-master/MN3_antispoof.xml --spf_model /home/adesoji/light-weight-face-anti-spoofing-master/MN3_antispoof.mapping.xml --cam_id 0 --config config.py;

i get the error

[ WARN:0@0.357] global /io/opencv/modules/videoio/src/cap_v4l.cpp (902) open VIDEOIO(V4L2:/dev/video0): can’t open camera by index Traceback (most recent call last): File “demo.py”, line 137, in <module> main() File “demo.py”, line 122, in main assert cap.isOpened() AssertionError

please how do i rectify this